We have a 5.1 home theater system and it works really well, except for the fact it only accepts stereo sound as input (it includes a DVD player, so it can play surround sound, but only if it's on a DVD).Now the DVD player/IR receiver/settings box seems to contain all the logic and controls and is connected with one big 25-pin cable to the subwoofer/transformer/amplifier box. So it should be possible to remove the DVD box from the cable and plug in a microcontroller instead, right?

But before I blow up either or both devices or electrocute myself, I'd like to learn what kind of signals I may produce on the I/O pins or what resistors, capacitors and wiggly wires must be added to the circuits. Or even how to measure along the cable during normal use to find out what signals to produce.

Here's the diagram of the main bord on the amplifier, hoping it's valuable to someone who knows electronics:

schematics from the internet and edited a bit

The 25 pins on the left go straight into the cable, while the 6+4+3 pins on the right feed to the speakers (of which the subwoofer wires stay inside the big box, because it's also the subwoofer)

To demonstrate how little I know about (consumer) electronics: I don't know whether the amp will get damaged (or simply won't power on) if the SYS6V is unused.[pseudoedit]...on the other hand, no one else can know either, because diagram B is not shown.

The various AC1,2,3,4 and FL1,2 are AC power (from different taps on the transformer), sent from this module to whatever your cable connects to.

The sys6v is DC power, also sent from this module to the downstream module(s). I presume that the downstream modules do their own downconversion to 5V or 3.3V. Pcont, sync and help must be switch signals to turn power on/off and register whether the power is working.

DC_det is connected to something that looks like an active filter stage for the subwoofer. This DC-det might be a signal to filter out (or just recognise) a DC component from the subwoofer signal. A DC component would damage the woofer. But that's a total guess.

The rest look like standard sound signals for the various channels. You should measure them (with an oscilloscpe preferably) at full volume to find out their peak voltage, you can them replace them by other sound signals as long as you don't exceed that voltage. Or experiment with weak signals first.

Zamfir wrote:The various AC1,2,3,4 and FL1,2 are AC power (from different taps on the transformer), sent from this module to whatever your cable connects to.

Do you think it would hurt the power supply if these are unused? (I have no use for AC power)

Zamfir wrote:The sys6v is DC power, also sent from this module to the downstream module(s). I presume that the downstream modules do their own downconversion to 5V or 3.3V. Pcont, sync and help must be switch signals to turn power on/off and register whether the power is working.

Would measuring the pcont/sync/help voltage compared to both the DC ground and the 6V work to find out what signal I should produce (to act like the system is operating normally)? (I assume they'll simply be shorted with the 6V or ground?)

Zamfir wrote:DC_det is connected to something that looks like an active filter stage for the subwoofer. This DC-det might be a signal to filter out (or just recognise) a DC component from the subwoofer signal. A DC component would damage the woofer. But that's a total guess.

Again measuring the voltage compared to the DC ground? While toying with Audacity (high frequency, low frequency and simply an offset) this time.

Zamfir wrote:The rest look like standard sound signals for the various channels. You should measure them (with an oscilloscpe preferably) at full volume to find out their peak voltage, you can them replace them by other sound signals as long as you don't exceed that voltage. Or experiment with weak signals first.

I can probably get my hands on an oscilloscope, but if not, can I use a regular multimeter too? Or the ADC (0-1.8V range, >1MHz rate) on my beaglebone?

It'd be great if they are "standard sound signals", because that sounds like I can simply plug the wires into the back panel of a desktop computer. (saves effort getting an expansion board, USB-audio or a whole different microcontroller or getting DACs or trying PWM)

I wouldn't expect problems from leaving the AC circuit unused. The various pcont, help, DC-det signals might be tricky. They probably prevent the amplifier form turning on too early, to prevent loud pops during startup. Check if the CD unit generates these signals, or only receives them. So feed the CD player with 6v and ground, but leave the other wires detached. Ccheck which side of those wires is pulled high and low. The other side of the wire is likely floating. That is, if you compare them with 6v they show zero difference, but if you compare them to ground, you also measure zero difference.

I would expect the sound signals to be regular 'line level' signals, usually with RMS voltage of around +-1V at max volume. But it's an internal line, so they might use whatever voltage they want. They can simply adjust the gain of the amplifier to adjust. If you have a multimeter with AC voltage, and you play a pure sine wave, you should get a good measurement.

EDIT clarification: the risk here is that the amplifier has an unusually high gain, so even a low sound-volume signal (low voltage) from the sound card already pushes the amp or speakers beyond their max. You might just try it, at a very very low sound-volume setting at first. Then slowly turn it up, your ears will warn you .

Be careful with the outputs of a sound card if it's a 'headphone' output. A regular 'line level' signal cannot deliver much current (high impedance). But a headphone output is supposed to drive a small speaker directly, so it can deliver a reasonable current, and has usually has a tad higher voltage as well. An official line-in to a stereo is typically designed to accept such a signal, but an internal link might not. I suspect it would work fine, just not 100% sure.